Applicants claim priority under 35 U.S.C. xc2xa7119 of Russian Application No. 2000111933 filed May 16, 2000. Applicants also claim priority under 35 U.S.C. xc2xa7365 of PCT/RU01/00193 filed May 15, 2001. The international application under PCT article 21(2) was not published in English.
This invention relates to construction of wells and is intended to produce vibrations in the string of tubes to reduce friction drag when it moves in the borehole, to free a stuck pipe and also to provide vibrational action on liquids that are filling the well.
A method is known to excite elastic oscillations in a well and a device therefor [USSR Certificate of Authorship no. 953183, 16.09.1980]. It comprises a moving element in the form of a ball that is placed into a string of tubes in a well, which ball transversely oscillates when pumping a liquid and due to this it hits the wall of the said tubes and thus transfers the oscillations to the string of pipes. The axial motion of the ball is limited by a support that is rigidly fixed at some location inside the tubing string.
These method and device can be considered as a prior art analog to the proposed invention. One can refer to the following general shortcomings of this analog:
Action of the device is localized in certain cross-section of the string of pipes since there is a stopper of axial motion of the working element;
The support (stopper of axial motion) made in the form of a grid adds surplus hydraulic resistance and induces vortexes in the fluid flow: the first results in additional losses of hydraulic power, the second affects stability of the knocker operation.
A device is also known [USSR Certificate of Authorship no. 1051233, 30.07.1982] for cementing of a casing pipe in a well which device comprises a string of pipes having a cement baffle collar at its shoe and which is filled with working fluid, and a driving plug with a vibrations generating mechanism placed on it. This the vibrations generating mechanism is equipped with a float fixed on it which float can be separated from the said driving plug once it sets on the cement baffle collar. The vibrations generating mechanism consists of a battery, switching unit and a float.
This device works as follows. After the computed volume of cement slurry was pumped into the casing pipe, a driving plug is run into it carrying the vibrations generating mechanism, battery, switching unit and a float. The switching unit turns on the vibrations generating mechanism when the cement rises in the annulus to a required level, for example, when cement level in the annulus and depth of said mechanism coincide. While this mechanism moves down to the cement baffle collar and then up to the well mouth it excites in the casing pipe the elastic oscillations which are transferred into the annulus where they act on the cement slurry there.
Due to this device a method is realized of vibrational action on a string of pipes which method comprises placing into the pipe string of a self-sustained mechanism for vibrational action, pumping into the string of pipes of working fluid and transportation with this fluid of the said mechanism for vibrational action and simultaneous exciting of transverse vibrations in the string.
This device and method which it implements are the most relevant to the proposed ones by technical realization and therefore they are selected as a prototype.
Main shortcomings of the prototype are as follows:
complexity of the vibrations generator design and a need in autonomous power supply;
narrow specialization, i.e. it is intended for vibrational action during cementing of casing pipe with a driving plug and float valve;
possibility is absent to controllably vary the intensity of oscillations because it is determined by the executive mechanism and said autonomous power supply;
vibrational action can be performed only single time starting from the moment of separation of the vibroactuator from the driving plug and only in one directionxe2x80x94from bottom hole to mouth of the well.
The proposed method of vibrational action on a string of tubes in a well comprises placement in the tubing string of an autonomous mechanism of vibrational action, pumping into the tubing string of a working fluid and transporting with it of the mechanism of vibrational action and simulataneous exciting by it of transverse oscillations of tubes in the string, where direction of transportation of the mechanism of vibrational action, frequency and amplitude of the tubes oscillations, and duration of the treatment within particular depth interval in the string of pipes are controlled by the working fluid pumping rate. Before placement of the mechanism of vibrational action into string of pipes a reference washing liquid pumping rate shall be firstly determined at which the transportation speed of the mechanism of vibrational action is equal to zero. The direction of transportation of mechanism of vibrational action is set in respect of this reference pumping rate, i.e. at higher working fluid pumping rate values comparing the reference one the mechanism of vibrational action is transported down to the bottom hole, and at working fluid pumping rate values lower than the reference one the mechanism of vibrational action is transported up to the well mouth. And vibrational treatment of selected interval of the tubing string is performed at either the working fluid reference pumping rate or higher or lower pumping rates, or at alternating such working fluid pumping rates.
The first embodiments of the device for vibrational action on a string of tubes in a well comprises the tubing string filled with the working fluid and a mechanism of vibrational action on the tubing string which mechanism is made as an element with positive floatability in said working fluid which element closes 0.85-0.98 of the tubing string cross-section area and can freely move in flow of the working fluid. In particular, mechanism of vibrational action can be made in form of a hollow ball which rigid casing is filled with a gas.
Thickness of wall of the hollow ball can be determined from condition of its floatability by the following formula:   t  ≅            1      3        ·                  ρ        liq                    ρ        met              ·    R  
where:
xcfx81liqxe2x80x94specific gravity of the working liquid;
xcfx81metxe2x80x94specific gravity of ball casing metal;
Rxe2x80x94radius of a ball.
Hydrodynamic force acting on the ball in working fluid flow can be derived from the following equation:   F  =      2    ⁢          πρ      liq        ⁢          v      2        ⁢          R      2        ⁢          {                        ln          [                      1                          1              -                                                (                                      R                                          R                      T                                                        )                                2                                              ]                -                              R            2                                R            T            2                              }      
where:
xcexdxe2x80x94speed of liquid flow in the tube;
RTxe2x80x94inner radius of the tube.
This force will keep the hollow ball suspended at same position and have it transversely vibrating, which vibrations will be transferred via hits on the wall to the string of pipes. Frequency and strength of these hits will vary depending on degree of closing of the tube cross-section. When speed of the fluid flow is increased the oscillating ball will move down, and when speed of the fluid flow is decreased the oscillating ball will move up counterflow.
As the laboratory test shown the hollow ball radially oscillates and the plane of vibrations rotates.
Another embodiment of the device for vibrational action on a string of tubes in a well comprises the tubing string filled with the working fluid and a mechanism of vibrational action on the tubing string which mechanism is made in form of a ball which closes 0.85-0.98 of the tubing string cross-section area, and a ball support made in form of a transverse bar or cross which support is rigidly connected with a cylindrical coil spring placed below the ball and having sliding fit to the tube, and force of the spring is selected accounting the axial load due to weight of the ball and hydraulic pressure by the flow presenting at the said working fluid reference pumping rate.
When the fluid flows around the ball the ball transversely oscillates and due to hits excites vibrations in the tube. To move the mechanism into another location downflow the fluid pumping rate shall be reduced below the reference value. Due to this the spring is released and the knocker, which continues to operate, moves toward bottom hole. For reverse motion of the knocker mechanismxe2x80x94to the well mouth, direction of working fluid flow shall be changed to opposite.